The visual brightness, or intensity, of an oscilloscope display is dependent upon several factors, including cathode-ray tube beam current, phosphor writing rate, sweep rate, signal repetition rate, and the functional abilities of the eye of an observer. In particular, in repetitive-sweep operating situations wherein the sweep repeats at a rate higher than the critical fusion frequency of the eye, which is the frequency at which a flicker can no longer be discerned, light levels of the individual sweeps are integrated by the eye so that the average brightness perceived is proportional to the ratio of the time the sweep trace on the viewing screen to the time it is off the screen. Hence, the brightness of the display becomes dependent upon the timing of trigger signals which initiate the sweep and the setting of the sweep rate switch. Generally speaking, application of a lower repetition-rate signal to the oscilloscope vertical input causes a lower duty cycle and therefore requires a manual adjustment of the beam intensity to increase the display brightness, and application of a higher repetition-rate signal causes a higher duty cycle and therefore requires a manual adjustment of the beam intensity to reduce the display brightness.
Attempts to solve the foregoing problem have been inadequate. Typically, the prior art involves an averaging circuit which responds to a sweep sawtooth or gate signal to generate a control signal, which signal is then applied to the biasing circuit of the cathode-ray tube to change the electron beam current when changes occur in the duty factor of the sweep signal. However, the control signal voltage, which is added to a fixed unblanking voltage level, does not provide a first order correction of beam intensity. For example, it can be seen that if the control voltage is very small compared to the unblanking voltage, halving the duty cycle does not result in the desired doubling of the beam current to maintain a constant intensity. Instead, the overall bias voltage change is very slight, resulting in an almost imperceptible correction of brightness.
Another problem associated with averaging circuits of the prior art automatic intensity circuits is that when extremely low-repetition-rate signals are applied to the vertical amplifier input, long time periods exist between triggered sweeps. After the termination of a sweep, the averaging capacitor begins to discharge so that when a new sweep is initiated, little or no correction at all is available. Depending upon the sweep rate and the ability of the averaging circuit to respond, the result may be either a sweep trace which begins bright and then dims as the trace progresses, or a sweep trace which simply stays bright. Further, for oscilloscopes having dual time bases which are alternatively displayed, one trace may be normal intensity while the other may be bright.